The invention relates to the mixing of particulate materials and more particularly but not exclusively to a method of mixing particulate materials with a given mixing ratio as part of a process requiring such materials and to an apparatus for performing the method. As used herein the term "particulate" does not limit the particles in the materials to any specific size or range but is merely to distinguish the materials from fluids or solid masses in one piece.
Difficulties are likely to be experienced in connection with the mixing of particulate materials having different specific gravities and/or different forms of the particles to get a given mixing ratio more particularly in those cases in which the paramount objective is not to produce an overall mixture containing given proportions of the individual components but rather to make certain that the distribution of the components in the mixture is as even as possible so that even if some fractions are relatively small the desired mixing ratio is in fact adhered to and/or if the mixing operation is to take place in a more or less continuous process. Even if it proves feasible to supply the amounts of the starting materials in the desired mixing ratio at least substantially continuously to a given position, there is bound to be a certain distance between the metering units supplying the different components of the mixture, or between the outlets of the units, so that the starting components will be separated from each other in space accordingly. It is not in fact possible for the components separated in this way to be supplied at one point by using a sort of hopper device because the components with their different specific gravities and other different properties may well adhere to a different extent and more specially in an uncontrolled manner to the wall faces or baffles of such apparatus so that there will be fluctuations in the mixing ratio. Mechanical mixing devices such as mixing drums, agitators, or the like also prove unsuccessful because with such differing properties of the components they are not able to mix the components evenly; a further telling point is that the transport from the mixing plant to the processing zone is likely to cause a further uncontrolled change in the mixing ratio.
In order to make the position clearer, the aim of the invention will now be elucidated on the basis of a practical example without this however limiting its field of application.
Synthetic resin components, now specially in the form of comminuted resin waste, may be supplied to a heated roll crusher and plastified there under the effect of heat and pressure so that such resin may then be immediately made into articles or, on the other hand, granulated. The components then supplied together may be quite different, as for example resin fragments produced by a cutting mill, plastic foil broken down into the form of flakes, or resin in powder form as well, for example for improving the quality of regenerated material.
To make certain that the plastified resin coming from the crushing rolls may be produced with an even quality it is necessary to see that the mixture of the different components is supplied to the rolls with a constant mixing ratio. For optimum operation of the rolls the component should be applied to them in the form of an evenly distributed, coherent, thin or single-thickness layer to make certain that the component is held fast by the foil coating of plastified resin on the roll and is successfully moved along into the crushing or squeezing zone. Therefore the specific problem to be solved is to supply different components with a given mixing ratio, that will vary from case to case, in the form of a relatively thin layer where processing is to take place and furthermore to see that the given mixing ratio is adhered to in the layer as well. Expressed generally, it is therefore possible to say that one aim of the present invention is to devise a method for mixing particulate mixture components with a given mixing ratio as part of a process requiring such materials such that the mixing ratio is adhered to even in the case of small amounts of the components till the components are fed into the means where such further processing takes place.
Because it is not possible to avoid separation of the components of a mixture during transport in containers and/or during temporary storage, the object of the invention is limited to improving the mixing operation as a part of a process including other operations.
In order to effect these and other objects of the invention, the components are separately metered out by means of a metering device operating at least substantially continuously and are distributed from the outlet of the metering device in a free fall onto a level, planar conveyor belt and are supplied in a further free fall from an output end of the belt into a further processing zone, the components preferably being distributed onto the conveyor belt in superposed layers.
By suitably matching the output rate of the metering units and the speed of the conveyor belt it is possible to produce a correspondingly thin layer which nevertheless keeps to the desired mixing ratio on the conveyor belt. Since the belt is moved on the level and in a planar form, there is no change in the position of the layer of component before it is discharged from the belt. Since the component passes from the discharge end of the belt to the further processing zone, it will arrive thereat with the desired mixing ratio as well
Because furthermore the component is distributed onto the conveyor belt in a thin layer, it is quite a simple step to remove metallic impurities from it on its way from the metering unit to the discharge end of the conveyor belt, such impurities resulting from wear of the metering units or otherwise finding their way into the layer of material as a component in the form of waste resin as supplied.
Therefore in keeping with a further useful development of the invention, the component located on the conveyor belt is freed of metal particles by a magnetic separator.
A further aim of the invention is to devise an apparatus for performing the method.
In keeping with the invention such an apparatus comprises a conveyor belt guided on the level and in a planar form as far as a point over the further processing zone to be supplied with the mixed components, a number of metering units for particulate components placed one after the other in the direction of conveying, such units operating at least substantially continuously, so that the component discharged therefrom and falling freely arrives on the conveyor belt generally over its full width.
For undertaking the method it is desirable that the separate components be metered out as continuously as possible. This is made possible by a particularly convenient development of the invention in keeping with which there is at least one ribbed roll in the metering passage of each metering unit between a feed port and an outlet port, such roll being able to be rotated about an axis running through the said metering passage transversely, and being furnished with shallow groove-like depressions placed successively in the circumferential direction and separated from each other by helical ribs from one axial end thereof to the other.
Because of this the separate depressions only take up a relatively small amount of component, it being possible by having a suitably large number of depressions to provide for a generally continuous discharge of the material. This effect is even further enhanced because the depressions run helically. This leads to a scattering effect in the direction of the axis of the ribbed roll and the discharge of component starts for example at one end of the ribbed roll, while it terminates at the other end of the ribbed roll.
Since the mixing method of the present invention lends itself particularly well to the mixing of components that are different in nature, as for example components with a granular, powder-like, lamellar or flake-like structure, it is particularly useful if the metering units may be adapted to the material of different components or may be adapted for this by substitution.
As an example of this, in keeping with a beneficial further development of the invention, the metering unit comprises two ribbed rolls arranged to be rotated about parallel axes synchronously in opposite directions, such rolls occupying the metering passage and having ribs adapted to mesh with each other like helical gearing, the distance between the axes of the rolls and the angle between the rolls as related to the circumference being adjustable.
This makes it possible to shut off the metering passage completely so that there is not the least chance of components leaking out, this being accomplished by turning one of the two rolls in relation to the other so as to make meshing engagement with the sides of the ribs thereon, whereas for metering a granular component a space is maintained between the ribs, corresponding to the maximum grain size, in the engagement zone.
If at least a part of the components to be mixed consists of resin or plastic foil comminuted into a flake-like form, there is a danger of the foil-like components fouling the ribs and the housing delimiting the metering passage so that the component becomes heated by friction and causes a stoppage of the metering unit with the result that operation may only be resumed after the metering unit has been cleaned. It is more specially in the case of operation of crushing rolls as noted that a given mixing ratio should be adhered to as closely as possible and furthermore continuous running of the apparatus is to be ensured.
To this end a further advantageous form of the invention is such that only a single ribbed roll is placed in the metering casing and the metering passage is limited by a knife that is fixed in relation to the ribbed roll so that its knife edge is adjacent to the roll, the outer edges of the ribs running towards the knife edge being designed as cooperating cutting edges.
In the event of large pieces of foil making their way through the comminuting means when comminuting foil-like synthetic resin, such pieces will not be taken up in the shallow, trough-like depressions, if they are very large, so that they will remain within the feed opening of the metering unit and may be removed therefrom. Smaller lumps, which are not yet in the desired flake form, are taken up in the trough-like depressions and are moved as far as the cutting zone at the knife where they are comminuted so that such foil-like component is not able to jam and interfere with continuous operation with a constant mixing ratio.
Preferably there is a guide face or lining that is parallel to the axis of the ribbed roll so as to delimit a feed duct in the direction of running of the ribbed roll so as to run towards the cutting gap between the knife and the ribbed roll, such guide face being adjustable in relation to the radial plane, containing the cutting gap, in a radial direction so that the metering unit may be adapted for the processing of foil component with a small cutting gap or the processing of a more typically granular component with a larger cutting gap.
Further advantageous and convenient forms of the invention will be seen from the claims.
Using the following account a detailed explanation will be given of working examples of the invention to be seen in the drawings.